1. Field of the Invention
This invention generally relates to a solid state laser and more particularly to a Q-switched solid state laser employing an acousto-optical element for controlling a Q-switch.
2. Description of the Related Art
Generally, it is preferable that a Q-switched solid state laser used in a technical field of micro-processing employed in a semi-conductor manufacturing process or of precision optical metering can generate pulse laser light which excel in a "beam pointing stability" when high peak power is applied to the laser, and also excel in a good "pulse-to-pulse stability". Incidentally, in the instant application, the term "beam pointing stability" is used to represent to what extent beam spots of laser light can stably be formed at the same position. Further, the term "pulse-to-pulse stability" is used to represent to what extent pulses of the same intensity can stably be generated.
A conventional Q-switched solid state laser employed in the technical fields as above described is disclosed in Japanese Unexamined Patent Publication (Kokai Tokkyo Koho) Official Gazette No. 63-168063 (U.S. Pat. No. 4,761,786 Aug. 2, 1988).
As shown in FIG. 2, a front mirror 2 and a rear mirror 3 forming resonance mirrors (i.e., cavity mirrors) of a resonator are respectively placed at both sides of a laser medium 1, which is formed by an Nd:YAG laser rod or an Nd:YLF laser rod, in this conventional Q-switched solid state laser. Further, an acousto-optical element (AOM) 4 for Q-switching is inserted between the laser medium 1 and the front mirror 2. Moreover, converging lenses 5 and a laser diode 6 for generating pumping laser light Lr are placed at the outside of the rear mirror 3 (namely, at the left side of the rear mirror 3 as viewed in this figure) in this order. Incidentally, the front mirror 2 transmits part of resonance laser light L and is used to extract oscillation laser light L' to the outside. Further, the rear mirror 3 transmits almost all of the pumping laser light Lr and performs nearly total reflection of the resonance laser light L.
In the above described conventional laser, the pumping light Lr is converged by the converging lenses 5 and is incident on an end surface of the laser medium 1 to pump the laser medium 1. As the result, the resonance laser light L is generated. At that time, a diffraction grating is caused in the AOM 4 for Q-switching by applying high frequency power to the AOM 4 by means of a drive device (not shown), so that a loss is inflicted on the resonance laser light L. Thus no laser oscillation occurs and pumping energy is stored in the laser medium 1. Under such a condition, if the high frequency power applied to the AOM 4 is suddenly cut off, the diffraction grating disappears and a resonance loss is decreased. As the result, the stored energy is released at a sitting and a laser pulse L' is put out.
The conventional Q-switched solid state laser described above does not need cooling water and a lamp employed as a broad-band pump source because of employing a laser diode as a pump source. Therefore, there is no influence of noises originated from the cooling water and the lamp on the laser. Moreover, there occur hardly thermal problems. Consequently, compared with the conventional laser of the type which employs a lamp as a broad-band pump source, the conventional Q-switched solid state laser described above excels in the "beam pointing stability" and the "pulse-to-pulse stability".
It, however, has turned out that the conventional Q-switched solid state laser does not necessarily have sufficient "beam pointing stability" and "pulse-to-pulse stability" to such an extent as become required recently.
According to the study by Inventor of the instant application, there is a factor affecting the "beam pointing stability" and "pulse-to-pulse stability" of the laser other than the induced noises and the thermal influence owing to the lamp employed as a pump source.
Namely, when high-frequency power is applied to an acousto-optical element for Q-switching, a gentle thermal gradient is caused in the acousto-optical element due to heat generated by the acousto-optical element itself and a connection portion used to connect the acousto-optical element with a lead for supplying power thereto. The thermal gradient gives rise to gradual change of refractive index in the acousto-optical element. As the result, there occur distortion of the wave front of an ultrasonic wave propagating through the acousto-optical element, a positional drift and distortion of the wave front of a transmitted wave. This has bad effects on the "beam pointing stability" and the "pulse-to-pulse stability" of the Q-switched solid state laser. The present invention is accomplished to solve such a problem of the conventional Q-switched solid state laser.
It is, therefore, an object of the present invention to provide a Q-switched solid state laser which excels in oscillation efficiency, the "beam pointing stability" and "pulse-to-pulse stability" and can be formed compactly.